Automatic-frequency-control system for an oscillator



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Aug; V14, 1951 I E. H. HUGENHOLTZ 4,

' AUTOMATIC-FREQUENCY-CONTROL SYSTEM FOR AN OSCILLATOR Filed Sept. 50, 1948 VVVVI Ill" INVENTOR.

1 EDUARD HEBMAZVHUGHUZIOZTZ Patented Aug. 14, 1951 AUTOMATIC-FREQU'EN'CY-CON.TRIOIL SYSTEMFOBJAN OSCILLATOR Eduard Herman: Hugenholtz, .Eindli'onlen bletherlau'ds, assignor. to- Hartford National: Bank: and. llrust. Company, Hartford Conn trustee Application September30, 1948, Serial No. 52,084

Inthe Netherlands October- 13; 1947' 5 Claimsi- (01. 250 -36) 1; invention relatestc acircuitrarrangement comprising anoscillator of which the frequency-determining oscillatory circuit is coupled: toa grid-controlled-amplfiying tube which is connected as a variable reactanceand used for automatic frequencyeorrection (AFC-l on acontrol oscillation, the control grid-of the amplifying tube being supplied-withthe AFC control voltage and also with a negative grid-bias, which serves: tofix theworking point-of the reactance tube.

In control circuits comprising a reactance tube it is known to obtainthe-negative grid-bias required for this tube. by including a. resistance. shunted by a. condenser in.the cathodelead of the reactance tubes This method of obtaining negative gridrbias is in. certain cases. inconvenient in. connection with. theresultant negative: back-coupling. of the. reactance tube,

- Furthermore it-is known to derive the negative. grid-bias required for a reactanca tube froma. voltage divider which-is connected parallel to the anode voltagesupply and a tap: of; which. is connected to. the cathodeof. the reactance tube.

The. object of the invention isto provide, a more advantageous method of. obtaining the negative grid-bias. required for areactance tube.

Applicant has surprisingly found that, accord.- ing to the. invention, in AFC circuit-arrange:-

ments. of the type described, the negative gridbias: for the reactance tube'may be: derived from atcathode resistance included inthe cathode lead: of; the oscillator tube;

Despite the consequent; direct-current coupling of the oscillator and reactance tubes and the'frequency of the oscillations produced. being dependent upon the direct-current positionof the two tubes, it. is found that any disturbing oscillatory. phenomena do not occur. Evidently, the influence of the stabilising control oscillation predominates such a circuit. It. is possible.- to utilise the voltage loss at the cathode resistance of the oscillator tube also as a negative grid-biasvfor theoscillatortube. Under certain conditions, however, the negative grid:- bias of the oscillator tube may, as a result of the anode current: of. the reactance; tube, have a value such that the; oscillator: circuit does not start to oscillate upon switching-in the arrangement.

In order to. avoid tlris, itzisadvantageous-for the extremity of the grid leak. which isremote from1 the control grid .to be. connected-:to the :exa: tremity of the cathode resistance. which adjathe cathode of the oscillator tube.

A particular: advantage when. using. the invention residesinth'eifactthat variationin. the; work? ing: point oftheireactance tube which is; produced. in. knnwm circuits uporr anode-voltage variations maybe counteracted and this by choosing; the: potential of the phase shift. between the grid; and. anode voltages ofithe; reactance tube to: be such that the transit-time; variations in the. oscillator tube whichamaprovoked: by anode voltage varia- ;;tio-ns bringaboutia variation; in the oscillator:

frequency WhiChLDIOdl-IGGSi ashifti of the working. point of the reactance tube which is inverseto theshift of the. working pointiof the: reactance tube: as a result of the variation in anode current produ'eediupon anodevoltage. variations. and the: variation. in the negative; grid' bi'as of. thereactance tube which is derived therefrom,

In this connection. itmay be observed. thatit is common practice amplitying circuits, for exampleinaradio-raceivers; toi derivethe negative grid-bias of severalamplifying tubes connected in cascadefrema cathode-resistance which is common to;v the tubes In such circuit-arrangements the. consegueniw direct=currentt coupling. of: the tubes doesnot. involve-undesirable oscil latory phenomena owing: to the absence of: direct-current coupling between the amplifying tubes connectediin cascade;

The invention-is: also applicable with advantage to acircuit-arrangement of the indicated type in which. to; the; frequency-determining oscillatorycircuit; of the oscillator coupled a further ad:- justable reactance the value of: which. is. con;- trolledbys the: direct-output current of an: output electrode;- of. the reactancetube. However, in this case thepotentialofthephase shift required betweenthegridt-and anodevoltages of the re.- actance tube mustibe. chosento be such that the reactanne tube and the: further adjustable re -v actanceassist on'e' 'another'in; their operation so that the: potential the phase shift can no longer bechosen freely inorder: to prevent varia tion' the workin-g po-intof the reactance tube uponi anodeivoltagei variations'.

Thecimzeiition: will now be explained morefull-y by reference to the accompanying drawing, in which; Figures; 11. and; 2 represent circuit-arrangements accordingtthereto" without: and with. additional frequency correction Identical-a elements are indicated in the: figures by the same: referencenumerals.

In Fig.1, referenccnumerall indicates: a triodewhich is used; as anposcillator and of which the anode and thecontrol .griid are" connected: by way ofacoupling; condensers .2 and to: different extremities of a frequency-determining oscillatory circuit 4. This oscillatory circuit comprises a tuning condenser and a circuit coil 6, a tapping of which is connected to earth. A grid leak i of the oscillator tube is connected to the cathode.

The anode of the oscillator tube is connected via a choke coil 8 and a resistance-condenser circuit 9, Iii, which serves to smooth the anode voltage, to the positive terminal ll of the anode voltage supply, of which the negative terminal I2 is earthed.

Parallel to the frequency-determining oscillatory circuit 4 of the oscillator circuit is connected a pentode l3, which is connected as a reactance tube, for the purpose of automatic frequency correction on a control'frequency.

The anode of the reactance tube I3 is connected by way of a blocking condenser [4 to that extremity of the circuit coil 6 which is coupled to the oscillator anode, while this circuit coil has derived from it, in a manner which will be described later, the alternating control-grid voltage of the reactance tube which is shifted in phase by 90 with respect to the anode voltage. The cathode of the reactance tube is earthed via a condenser I5, so that the anode-cathode space of the reactance tube for the operating frequency is connected parallel to that part of the circuit coil 6 which is located between the extremity of the oscillator and the earthed tapping.

The anode of the reactance tube is connected via a decoupling choke l6 and a resistancecondenser circuit I'I, I8, which serves for decoupling, to the positive terminal ll of the anode voltage supply. 7

"According to the invention, the cathode of the reactance tube is connected to earth by way of a cathode resistance l9, which is included in the cathode lead of the oscillator tube and shunted by the condenser 15. The latter has a capacity value which is common for shunting cathode resistances, for example from 1 to 2 microfarads. Since the control grid of the reactance tube is connected to earth via a leak resistance 20, the negative grid-bias required for fixing the operating point of the reactance tube is thus derived from the oscillator tube circuit. If desired, the grid bias may be derived from a part of the cathode resistance by connecting the cathode of the resistance tube to a tap on the cathode resistance l9.

In order to obtain the control voltage required for the tube l3, connected as a variable reactance, for automatic frequency correction of the oscillations generated by the oscillator on a control oscillation, the oscillations generated by the oscillator are supplied via a blocking condenser 2| and a control oscillation supplied to the terminals 22 directly to a discriminator 23. The latter is so constructed as to supply a control voltage to the controlgrid of the reactance tube, the value and potential of which vary with the value and potential of the required frequency correction.

Transit-time variations in the oscillator tube provoked by anode voltage variations bring about variation in the frequency .of the oscillations generated by the oscillator. This results, with unvaried control frequency, in a variation in the control voltage supplied to the reactance tube such that the oscillator frequency again corresponds to the control frequency.

These anode voltage variations will, in addition, produce a variation in the anode current of the oscillator tube and hence of the voltage set up at the cathode resistance IQ of the tube, which results in variation in the negative gridbias of the reactance tube and hence of the working point of this tube.

In order to prevent the control range of the reactance tube from being greatly reduced in one sense or another upon anode voltage variations, the attended variations in the control voltage of the reactance tube and the shift of the operating point may be made opposite to one another.

To this end, according to the further invention, the potential of the phase shift between the grid and anode voltages of the reactance tube is suitably chosen, for which purpose in the example shown, for example the alternating control-grid voltage of the reactance tube may be derived across a phase-shifting network constituted by a resistance-condenser circuit 24, 25 from the grid side of the circuit coil 6 instead of from the anodeside thereof, as is usually the case.

The example shown in Fig. 2 is a circuit which is essentially identical with that shown in Fig. 1, only use being made of an additional frequency correction. Coupled to the frequency-determining circuit 4 of the oscillator l is an adjusting coil 25 which is connected parallel to a part of the circuit. coil 6. The regulating coil 26 is provided, jointly with a pre-magnetizing winding 2', which is included in series with the decoupling choke HS in the anode circuit of the reactance tube l3, on a common core 28. The variation in direct anode current provoked by variation in the control voltage supplied to the control grid of the reactance tube controls the pre-magnetization of the core 28 and hence the inductance of the adjusting coil 26, which brings about variation in the tuning of 'the frequencydetermining circuit 4. 1

The polarity of the inductance variation of the adjusting coil 26 is fixed with frequency correction by variation in the pre-magnetization of the core 28. Since forobtaining additional frequency correction it is required that the frequency correction produced by this adjusting coil and the'frequency correction produced in itself by the reactance tube should assist one another, it is not possible, as in the .-example shown in Fig. 1, to choose freelythe polarity of the phaseshift between the grid and anode voltages of the reactance tube in order to prevent variations in the operating point of thereactance tube upon anode voltage variations.

It is possible to utilize the voltage set up at the cathode resistance l9 or a part thereof also as a negative grid-bias for. the oscillator tube. In order to avoid, however, that upon switching-in the circuit the oscillator would not start to oscillate on account of undue negative gridbias as a consequence of the anode current of the reactance tube, thegrid-leak I of the oscillator is preferably connected to the cathode of this tube.

What I claimis:

1. An electrical circuit arrangement comprising wave generating means comprising a first discharge tube system having cathode, control grid and anode electrodes, and a resonant circuit coupled to said electrodes in regenerative feedback relationship to produce a wave having a given frequency, means to apply an energizing potential to said anode electrode, said wave generating means undergoing variations in frequency gears-ca discharge tube system proportional to the amplitude of current fiow through said first discharge tube system, means to couple the cathode and anode electrodes of said second discharge tube system to said resonant circuit, phase shifting means coupled to said resonant circuit for producing a quadrature voltage, and means to apply said quadrature voltage to said control grid of said second discharge tube system in such a phase that frequency changes due to variation in Value of the said energizing potential are compensated by reactance changes due to changes in said biasing potential.

2. An electrical circuit arrangement comprising wave generating means comprising a first discharge tube system having cathode, control grid and anode electrodes and a resonant circuit coupled to said electrodes in regenerative feedback relationship to produce a wave having a given frequency, means to apply an energizing potential to said anode electrode, said wave generating means undergoing variations in frequency in a given direction proportional to the value of said energizing potential, reactance variation means comprising a second discharge tube system having cathode, control grid and anode electrodes, a first resistor element connected in common to the cathode electrodes of said first and second discharge tube systems to thereby apply a biasing voltage to the control grid electrode of said second discharge tube system proportional to the amplitude of current fiow through said first discharge tube system, a second resistor element intercoupling the control grid and cathode electrodes of said first discharge tube, means to couple the cathode and anode electrodes of said second discharge tube system to said resonant circuit, phase shifting means coupled to said resonant circuit for producing a quadrature voltage, and means to apply said quadrature voltage to said control grid of said second discharge tube system in such a phase that frequency changes due to variation in value of the said energizing potential are compensated by reactance changes due to changes in said biasing potential.

3. An electrical circuit arrangement comprising wave generating means comprising a first discharge tube system having cathode, control grid and anode electrodes and a resonant circuit coupled to said electrodes in regenerative feedback relationship to produce a Wave having a given frequency, means to apply an energizing potential to said anode electrode, said wave generating means undergoing variations in frequency in a given direction proportional to the value of said energizing potential, reactance variation means comprising a second discharge tube system having cathode, control grid and anode electrodes, a resistor element connected in common to the cathode electrodes of said first and second discharge tube systems to apply a biasing voltage to the control grid electrode of said second discharge tube system proportional to the amplitude of current flow through said first discharge tube system thereby to vary the reactance of said reactance variation means in a given sense, means to couple the cathode and anode electrodes of said second discharge tube system: said. resonant: circuit, phase shifting means coupled to said resonant. circuit for producing; a\ quadrature voltage, and: means to apply said quadrature voltage to said control. grid of said second discharge tube system; in such a phase: that the reactance of said reactance variations means; is varied in a sense opposite to said given sense thereby to compensate for frequency changes due: to variations in value: of the said. energizing potential.

4. An electrical circuit arrangement comprising Wave generating means comprising a first discharge tube system having cathode, control grid and anode electrodes and a resonant circuit coupled to said electrodes in regenerative feedback relationship to produce a Wave having a given frequency, means to apply an energizing potential to said anode electrode, said wave generating means undergoing variations in frequency in a given direction proportional to the value of said energizing potential, reactance variation means comprising a second discharge tube system having cathode, control grid and anode electrodes, a resistor element connected in common to the cathode electrodes of said first and second discharge tube systems to thereby apply a biasing voltage to the control grid electrode of said second discharge tube system proportional to the amplitude of current flow through said first discharge tube system, means to couple the cathode and anod electrodes of said second discharge tube system to said resonant circuit, phase shifting means coupled to said resonant circuit for producing a quadrature voltage, means to apply said quadrature voltage to said control grid of said second discharge tube system in such a phase that frequency changes due to variation in value of the said energizin potential are compensated by reactance changes due to changes in said biasing potential, frequency detector means coupled to said resonant circuit to produce a control potential having a polarity and amplitude proportional to variations in frequency from said given frequency, and means to apply said control potential to the control grid of said second discharge tube to vary the reactance of said reactance variation means thereby to maintain the frequency of said wave substantially constant.

5. An electrical circuit arrangement comprising wave generating means comprising a first discharge tube system having cathode, control grid and anode electrodes and a resonant circuit coupled to said electrodes in regenerativ feedback relationship to produce a wave having a given frequency, means to apply an energizing potential to said anode electrode, said Wave generating means undergoing variations in frequency in a given direction proportional to the value of said energizing potential, reactance variation means comprising a second discharge tube system having cathode, control grid and anode electrodes, a resistor element connected in common to the cathode electrodes of said first and second discharge tube systems to thereby apply a biasing voltage to the control grid electrode of said second discharge tube system proportional to the amplitude of current flow through said first discharge tube system, a reactive element havin a reactance value proportional to the anode current of said second discharge tube system and being coupled to said resonant circuit to vary the resonant frequency of said resonant circuit proportional to the voltage applied to the control grid of said second discharge tube, means to couple the cathode and anode electrodes of said second discharge tube system'to said'resonant circuit, phase shift-- ing means coupled to said resonant circuit for producing a quadrature voltage, and means to apply said quadrature voltage to said control grid of said second discharge tube system in such a phase that frequency changes due to variation in value of the said energizing potential are compensated by reactance changes due to changes in said biasing potential.

EDUARD HERMAN HUGENHOLTZ.

file of this patent:

UNITED STATES PATENTS Number Name Date Rath Oct. 7, 1941 Usselman Oct. 13, 1942 

